1. Field of the Invention
The present inventon relates to a system for determining the instantaneous positions of a plurality of vehicles, particularly aircraft, traveling on or above a defined sector of the earth's surface.
2. Description of the Prior Art
Location of an aircraft in latitude and longitude by present-day air traffic control systems usually depends on direct radar interrogation. Radars typically scan only a few times per minute, because for each radial scanned the radar must wait for echoes out to the full range before transmitting the pulse for the next radial. Consequently, the frequency at which radar-derived position information can be updated is inherently limited. Radars are unable to detect aircraft beyond direct line-of-sight, and because of uneven terrain and the curvature of the earth many areas in which aircraft fly are unseen by radar, particularly if the aircraft is at low altitude. Although the range (i.e., the distance of the aircraft from the radar antenna) can in principle be measured quite precisely, precision of measurement of the azimuth (i.e., the bearing of an aircraft relative to North from a radar transmitter, usually expressed in degrees) depends on radar beam width and is relatively poor.
Measurement of the height of an aircraft cannot be made except very crudely by radar; therefore, even for the largest and best-equipped commercial jets, it is usually inferred from the local air pressure, corrected approximately by local barometric settings when those are known. Under the present system of air traffic control, the pressure measured by an aneroid barometer is converted to digital signals which are encoded and returned to Air Traffic Control (ATC) by way of the aircraft "transponder", a device which responds to radar impulses by returning a coded sequence of pulses. The existing system of altitude measurement is therefore inherently crude, and its usefulness for terrain avoidance in nonvisual instrument flight conditions (IFR) depends on accurate knowledge of the local barometer setting (which can change rapidly under certain weather conditions) and on the aircrew remembering to update the barometer setting frequently.
Because of the imprecise knowledge of height and azimuth, and the inability of radar to scan close to the ground, the air traffic control radar system cannot be used as a precision landing approach system in IFR conditions. It must therefore be supplemented by an entirely separate system, for example the ILS, which must be duplicated for each runway or airport.
For similar reasons, attempts to use the present system to provide warning of possible midair collisions have been unsatisfactory. At the slow radar scan rate, and with the large errors in measurement of height, azimuth and speed, the extrapolated paths of each aircraft are in fact expanding cones of uncertainty, so large in extent that many false alarms occur. In a typical moderate traffic airport region these "conflict-alert" warnings may sound ten or more times per day. Therefore, controllers tend to disregard them, having learned that most alarms are false. This has contributed to major air disasters.
The most dangerous navigational situation is flight in a mountain valley in nonvisual or IFR conditions. The present radar system can provide little help for that situation, because it cannot reach into a valley below its horizon-skyline.
Because of the many uncovered regions where radar does not reach, aircraft must navigate by still another, independent system. Of these, the most common is the Very High Frequency Omnidirectional Range (VOR) system stations to provide azimuth information. For aircraft so equipped, this is supplemented by Distance Measuring Equipment (DME) for distance from the ground VOR station. In the VOR system, a radio transmitter defines a narrow radial line which sweeps in a full circle around the transmitter many times per second. From it, special equipment on board an aircraft can obtain the bearing from the VOR transmitter. In this system azimuth is poorly measured, nominally with an uncertainty of several degrees, which converts to several miles at a typical distance of 60 miles from a VOR station. There are, in addition, many blind spots in this sytem, particularly at low altitude. Moreover, since most aircraft have to follow radial lines from VOR stations, typical air traffic routes are of zigzag form, covering greater than straight-line distance and thereby unnecessarily wasting fuel.
Distance Measuring Equipment refers to a special transmitter/receiver combination carried on board some aircraft. The transmitter sends out pulse-type interrogation signals which are received by military Tactical Air Navigation (TACAN) stations. TACAN stations are usually located at the same place as ("co-located" with) VOR stations. The TACAN station sends a reply signal which the airborne DME receives. From the elapsed time and the known speed of propagation of the radio signals, the aircraft DME computes the distance from the TACAN station. The principal drawback of this system is the complexity and expense of the airborne equipment required for interrogating the TACAN station and processing the reply signal; these factors render DME practical only for relatively expensive business aircraft, and for the larger and more sophisticated military and commercial aircraft.
The Nondirectional Beacon and the Instrument Landing System represent still further parts of the existing patchwork of air traffic control systems. The Nondirectional Beacon (NDB) is essentially a "homing" transmitter usable for nonprecision navigation and approaches. The Instrument Landing System (ILS), much more sophisticated, is implemented by special radio equipment provided for certain runways at some airports. The ILS actually consists of three separate radio systems (all unrelated to VOR, NDB or to any of the other systems described previously) for transmitting information to the aircraft relating to its left-right position and its angle vertically from the end of the runway (i.e., glideslope), and its horizontal distance from the runway. The radio information so transmitted must be decoded by special equipment provided for that purpose on board the aircraft.
Because the systems now required on an aircraft to enable it to measure its (a) height, (b) azimuth, and (c) distance from a VOR; its (d) height and (e) azimuth on ILS during a precision approach to landing; (f) its location relative to a Nondirectional Beacon (NDB); and (g) its distance from a runway on a precision approach are all different, an aircraft fully equipped for IFR flying requires a large number of different electronic units, all of them expensive and subject to failure. For that reason, only a fraction of all aircraft are equipped with even minimal blind-flying equipment. Only the largest, most expensive aircraft carry substantial redundant equipment for all of the many systems involved. Equipment weight is also a factor; on the smaller types of business aircraft normally equipped for IFR flying, the weight of the necessary IFR electronics carried is often as much as 5% of the aircraft useful load, and subtracts either from fuel or from payload.
Because the present system for precision approaches (Instrument Landing System or ILS) is unable to lead an aircraft except along a straight line, aircraft must line up for approach many miles out. This limits the ability of an airport to handle large traffic volume. Recognizing this limitation, the Federal Aviation Agency presently plans to require still another system, the Microwave Landing System (MLS), to overcome some of the deficiencies of ILS. This will further increase the on-board equipment necessary for IFR flying, and so further reduce the number of aircraft operators who can afford such flight.
A further problem in the existing system of monitoring and controlling air traffic relates to locating an aircraft in the event of a crash. At present, the so-called "emergency locator transmitter" (ELT) is relied upon for this purpose. The ELT is a battery-operated device, required by law for each aircraft, which is intended to begin transmitting a distress signal on crash impact. Most ELT signals are in fact false alarms, and in the event of an actual crash ELT antennas are often broken or covered by shielding debris, thereby rendering the ELT ineffectual. The air traffic control radar system is of little help in this critical situation, since a descending aircraft disappears below the radar horizon at the point when its situation becomes most serious.
A final problem in the existing air traffic control network is that of voice communications between aircraft and ground control. Such communications now depend on scattered transmitter-receiver locations across the country. This system, too, has blind spots, particularly at low altitudes or in mountainous terrain. In a given traffic area, it is common for many aircraft to communicate with air traffic control on the same frequency, forcing information to be exchanged on a "party line" basis and creating the danger that information intended for one aircraft will be erroneously acted on by another. During approach to landing, moreover, communications frequencies must often be switched manually as the aircraft passes from the jurisdiction of one ground controller to another, thus creating additional distraction for the aircraft's crew at a time when many other matters require their attention.
Various proposals have been made for supplementing or replacing the existing fragmented system of air traffic control and navigation with a unified, comprehensive system covering large areas of the earth's surface. One particularly ambitious attempt along these lines is the Navstar system, also referred to as the Global Positioning System (GPS), currently undergoing development in the United States by the Department of Defense. Military aircraft have navigational requirements entirely different from those of civilian aircraft. For military aircraft the cost of navigational equipment is a secondary consideration, the foremost being the ability to navigate easily in every part of the world and to avoid transmitting any signals that would reveal their whereabouts in a hostile situation. As presently envisioned, the Navstar system will employ a system of twenty-four satellites in three mutually orthogonal twelve-hour orbits about the earth (two polar and one equatorial). The satellites transmit unique identifying signals on a common carrier frequency received by the craft whose position is to be calculated. Based on the propagation times of signals from four of the satellites to the receiver on the craft, the location of the craft can be calculated from the known instantaneous positions of the satellites involved. Full implementation of this system will require, among other things, atomic clocks for providing timing synchronization to the necessary level of accuracy and complex computational equipment on the craft or at a central site accessible to the craft by a satellite link. The sophisticated equipment required for navigating with the Navstar system, particularly where the navigational computations must be carried out on board the craft, is likely to render this sytem inaccessible to most aircraft operators for cost reasons. Reference may be had to U.S. Pat. No. 4,114,155, for example, and to the references cited therein for a description of the Navstar system.
Other radio navigation systems employing artificial satellites in earth orbit have also been proposed. U.S. Pat. No. 3,665,464, for example, describes a system for high-speed aircraft position determination using three spaced antenna sites and a beacon responder aboard the aircraft to be located. The system is said to be usable in a ground-based configuration or in connection with a number of synchronous, near synchronous, or non-synchronous satellites. A beacon transmitter at one of the antenna sites interrogates the aircraft at a defined point in time using a discrete aircraft code or pulse group, in response to which the beacon responder on the aircraft transmits a reply signal which is received at all three antenna sites. A ground computer when calculates the aircraft position based on the interrogation time, the time a reply was received at each of the three antenna sites, and the known positions of the antenna sites. The aircraft position information thus calculated is transmitted back to the aircraft as part of the next interrogation signal. The problem of overlap between reply signals originating from different aircraft is handled essentially by initially determining the positions of all aircraft within the range of the system and thereafter interrogating the aircraft in order of their proximity to the beacon transmitter.
A somewhat different satellite-based radio navigation system is described in U.S. Pat. No. 3,384,891. In what is referred to as the "active" mode of operation, a ground station transmits time-spaced ranging signals to each of two satellites in synchronous or nonsynchronous orbits. The ranging signals each carry digital address codes identifying one of the satellites and the particular vehicle to be located. The satellites individually retransmit their respective ranging signals to the vehicle and also directly back to the ground station. Equipment carried aboard the vehicle repeats the two ranging signals retransmitted by the satellites and relays them back to the ground station through the respective satellites. Based on the measured differences between the arrival times at the ground station of the directly and indirectly retransmitted ranging signals associated with each satellite, the ground station computes the range of the vehicle from each satellite. Alternatively, a single nonsynchronous satellite may be interrogated at two known orbital positions to obtain the two range values. In either case, if the vehicle is located on the earth's surface, these two ranges define two circles of position intersecting at two points, one of which is the vehicle's position and the other of which is an ambiguity that is rejected based on an approximate knowledge of the vehicle's true position. If the vehicle is located above the earth's surface, a similar procedure can be carried out if the altitude of the vehicle is separately determined; alternatively, three (rather than two) satellites may be interrogated to obtain three range measurements to the vehicle, which permits calculation of a complete position fix, including altitude.
In the alternative "passive" mode of operation of the system described by U.S. Pat. No. 3,384,891, the ground station separately transmits the known instantaneous positions of the two satellites to the vehicles. Immediately thereafter, the ground station transmits ranging signals to each of the two satellites with anticipated propagation times such that the ranging signals are repeated and retransmitted by the two satellites substantially simultaneously. The retransmitted range signals are received by the vehicles with a time difference indicative of the range difference between the vehicle and the two satellites. This difference defines a hyperbolic surface which is resolved into a line of position for the vehicle if the vehicle is on the earth's surface or if its altitude is known. Repeating this procedure with different pairs of satellites produces intersecting lines of position which define the vehicle's position. The distinguishing feature of the passive mode is that no radio transmission is made from the vehicle, and its position is therefore not made known to others. A description of the above-described system in both the active and passive modes can also be found in a paper entitled "A Navigation System Using Range Measurements From Satellites With Cooperating Ground Stations", Journal of the Institute of Navigation, Vol. 11, No. 3 (Autumn 1964), at pp. 315-334.
U.S. Pat. No. 3,430,234 relates to a radio navigation system which employs a plurality of satellites in stationary (i.e., geosynchronous) earth orbit. In particular, six stationary satellites are evenly spaced about the earth in an equatorial plane for ensuring line-of-sight communication between a craft nearly anywhere in the world and at least two of the satellites. Each satellite carries a receiver for receiving identity-encoded interrogation signals generated by the craft to be located and a transmitter for transmitting signals synchronized with the received interrogation signals. The signals produced by the satellites in response to a craft-generated interrogation pulse are received by the craft and their time difference determined by craft-carried equipment to generate a hyperboloid which intersects the earth's surface (or, in the case of an aircraft having a known altitude, a spherical surface above the surface of the earth) to define a line of position for the craft. A second, intersecting position line is determined by measuring the round-trip transit time of an interrogation signal generated by the craft and relayed back to the craft by one of the satellites, thereby locating the craft position. Alternatively, the second position line is obtained by determining the sum of the round-trip transit times of the interrogation signal through the two satellites, which produces an elliptical position line that intersects the original hyperboloid-defined position line at the craft location. To prevent signal overlap at the satellites when a large number of craft are using the system, it is suggested that time division multiplexing may be realized by transmitting from one of the satellites an interrogation synchronizing signal which functions to ensure that the interrogation signals transmitted by the various craft sharing a common frequency occur in a predetermined sequence.
U.S. Pat. No. 3,544,995 discloses a further navigation system making use of one or more artificial earth satellites. In a first version of the system, a single satellite is used for relaying to a ground station aircraft position, identification and altitude information which has been generated separately by equipment carried on board the aircraft. The ground station receives and records the information generated by a plurality of aircraft for use in collision avoidance. In a second version of the system, signals coded with aircraft-identifying addresses are transmitted by the ground station and relayed to the identified aircraft via a pair of satellites. The aircraft carries a transponder which detects these signals and transmits a return signal that is relayed back to the ground station through the two satellites. Highly directive antennas are utilized at the ground station to separate the signals from the two satellites. The return signal includes altitude information derived from a radio or barometric altimeter aboard the aircraft. Based on the transit times of the signals relayed back to the ground station through the two satellites, a ground station computer calculates the position of the aircraft using the given altitude information. The position and aircraft identification information is then relayed back to the vehicle through one of the satellites. Overlap of return signals from different aircraft is prevented either by ensuring that the signals originated by the ground station for the different aircraft are sufficiently far apart to preclude overlap of the return signals, or by arranging the vehicle addresses in the ground station computer according to their distances from the satellite. In an exemplary system, six equidistant synchronous satellites are spaced around the earth's equator to allow coverage of all points on the surface of the earth up to geographic latitudes of .+-.75.degree..
Widespread acceptance of a satellite-based air traffic control and navigation system will ultimately depend upon the following four factors: (1) Precision of location of aircraft position, (2) the allocation of complex hardware which makes up the system as between aircraft, satellites, and ground station, (3) the extent to which the system can resist overloading or "saturation" even in the case of vigorous growth in the number of aircraft monitored by the system, and (4) the extent to which the system is adaptable to fully automatic or "pilotless" flight.
As to the first of these factors, all large-scale general-purpose position-measurement systems depend directly or indirectly on the measurement of time intervals, converted to distances through multiplication by the velocity of light. The precision of time-measurement is proportional to the bandwidth that can be allocated to the measurement. Any system that can only function effectively by subdividing the available bandwidth into a large number of narrower-band channels (for example, to avoid system saturation) must therefore sacrifice precision of measurement of time and therefore, ultimately, of position.
As to the second factor, it is clear that the optimum allocation of system components is that which places the least sophisticated hardware in the individual aircraft and the most sophisticated hardware at the ground station, since the latter represents what is essentially a one-time expenditure while the former represents an expense that is incurred for every aircraft that can use the system. An air traffic control system is of little use unless it is sensitive to all aircraft, and such capability will not be affordable for all aircraft unless the required on-board equipment is very simple and inexpensive. From a reliability standpoint, moreover, the complexity of the hardware carried by the satellites should also be minimized, since there are not readily accessible for repairs once they have been placed in orbit.
With respect to the third of the factors mentioned above, the dramatic escalation in commercial and private air traffic over the past few decades makes it clear that any system which is adopted must be able to handle a ten- or even hundred-fold increase in air traffic over present levels without a serious degradation in performance.
Finally, in view of the already critical nature of air traffic density over major urban centers, and the small margin for error at the high speeds attained by modern aircraft, it is inevitable that resort will be had to fully automatic or pilotless flight at least to some degree in the not-too-distant future. This will place exacting demands on the performance of the system; response times that are sufficient for passive monitoring purposes may be wholly unacceptable if the system will also be required to control the movements of the aircraft being monitored.
Implementation of a new air traffic control system is a major undertaking, typically requiring several decades. Once in place, an ATC system is expected to remain operational for several decades more. The system must therefore be designed with a great deal of foresight, since premature obsolescence can mean the loss of a large investment in labor and equipment. In light of what has been said above, it is clear that a fully satisfactory air traffic control system will have to be (1) applicable in at least a rudimentary form to every aircraft flying, in order to render the ATC effective in collision avoidance, (2) capable of expansion, without saturation or significant reduction in effectiveness, to match the enormous increase (perhaps by a factor of 100) in the total number of aircraft that could come about within the next several decades, and (3) readily extendible, without major retrofitting or scrapping of system components, to fully automatic or "pilotless" flight. It is only by satisfying all of these criteria, which have heretofore been seen as inherently incompatible or mutually exclusive, that an air traffic control system can be assured of practical implementation and freedom from premature obsolescence.
Each of the systems so far proposed for carrying out satellite-based air traffic control and navigation falls short with respect to one or more of the foregoing criteria which, it is to be emphasized, must all be met simultaneously if the system in question is to have general applicability and long-range utility. The Navstar system, with its dependence on complicated position-computation equipment aboard the individual craft, is not affordable except for military and the more expensive business and commercial aircraft. These constitute only a few percent of the total air fleet. Of the remaining systems, those which call for discrete interrogation of the individual aircraft using pre-assigned address codes or the like also place an undue equipment burden on the individual craft, since those craft must then carry special equipment for recognizing their unique addresses before responding to a particular interrogation. Such equipment must be duplicated for each aircraft using the system.
Discrete interrogation of individual aircraft has been seen as necessary in prior art systems for a number of reasons, important among these being the need to ensure that the signals returned by the different interrogated aircraft do not overlap at the receiving site. Even with discrete addressing, however, the overlap problem is not necessarily solved, since the return signals from aircraft at different distances will not necessarily arrive back at the ground station in the same order in which the aircraft were interrogated. Thus further makeshift solutions are required, such as the suggestion in the above-cited U.S. Pat. No. 3,665,464 that the positions of all aircraft within range of the system be initially determined and thereafter interrogated in order of their proximity to the transmitter. Clearly, the relative positions of the aircraft tracked by the system will be constantly changing, requiring continuous reshuffling of information in the system's computer memory. An alternative solution to the problem, suggested in U.S. Pat. No. 3,544,995, is to ensure that the interrogation signals originated by the ground station for the different aircraft are sufficiently far apart to preclude overlap of the return signals. This would require that the ground station wait for return signals from aircraft located at the maximum range of the system before transmitting the next interrogation signal, severely increasing the time required for a single inventory of all the aircraft tracked by the system. As a consequence, for any sizable number of aircraft, the frequency with which the position of any given aircraft could be fixed is far too low for the fully automated flight applications referred to previously.
Along similar lines is the solution proposed in U.S. Pat. No. 3,430,234. In this system, it will be recalled, interrogation is carried out by the craft itself and responded to by the satellites whose positions are used to fix the position of the craft. To prevent signal overlap at the satellites when a large number of craft are using the system, it is suggested that one of the satellites may transmit an interrogation synchronizing signal following which each craft is assigned a limited period for carrying out its distance measurement functions. During this period, no other interrogation takes place from craft sharing the same carrier frequency. The intended effect, therefore, is to time-division multiplex the interrogation signals transmitted by the various craft by ensuring that they occur in a predetermined sequence after the synchronizing signal. The necessary time "window" which must be assigned to each aircraft, however, is said to be equal to the maximum value of the possible range of variation of the signal transit time for the complete interrogation path, or twice the delay equivalent of an earth's radius. The problem with this expedient, then, is essentially the same as that encountered previously: for realistic numbers of aircraft, the cycle time of the system becomes intolerably large for effective air traffic control and automated flight applications. Perhaps in tacit recognition of this fact, U.S. Pat. No. 3,430,234 appears to contemplate use of the system only as a navigational aid for the individual craft, using craft-borne computational equipment to carry out all the necessary distance and position calculations, rather than as a centralized air traffic control system. The patent does suggest, however, that the cycle-time limitations of the system may be avioded by providing a number of different operating frequencies, and then assigning a limited number of aircraft to each frequency on the time-division multiplexed basis described earlier. This expedient, of course, merely substitutes one problem for another, since for large numbers of aircraft the number of channels required would be large, so that the bandwidth for each would be reduced and the precision of position measurement for all aircraft would be correspondingly degraded.
One attempt to avoid discrete addressing of individual aircraft, albeit not in connection with a satellite-based position determination system, is described by O'Grady et al. in a paper entitled "ATCRBS Trilateration: The Advanced Airport Surface Traffic Control Sensor", published in AGARD Conference Proceedings (No. 188) on Plans and Developments for Air Traffic Systems (Cambridge, Massachusetts, May 20-23, 1975). The purpose of the proposed system is to locate and identify aircraft on the airport surface using three ground antenna locations placed around the periphery of the airport. An interrogation signal from one of the antenna locations causes a beacon transponder aboard the aircraft to produce an identity-coded reply signal which is bracketed by leading and trailing framing pulses for time measurement purposes. Differences in the arrival times of the reply signals at the three antenna locations enables the aircraft position to be calculated in two dimensions (i.e., on the airport surface) by hyperbolic methods.
In the system proposed by O'Grady et al., the interrogation signal is not coded with the identity of any particular aircraft and will theoretically elicit a response from any aircraft receiving it. Consequently, reply signal overlap at the ground receiving antenna locations becomes a serious problem, particularly in the critical situation that occurs when two aircraft are very close to each other. This, of course, is the situation in which accurate position monitoring is most needed. As recognized by O'Grady et al., the problem is not avoided merely by using a highly directional interrogation signal radiation pattern, since it is always possible that two or more closely spaced aircraft may be in the interrogator beam at the same instant. To prevent undesirable reply signal overlap, therefore, O'Grady et al. provide for temporary suppression of the aircraft transponders (i.e., inhibition of the transponders from replying to all valid interrogations for a fixed period of time) in response to the receipt of an appropriately coded suppression signal. The suppression signal is transmitted in a steered (i.e., moveable) manner from two of the ground antenna locations with a deep notch or null in its radiation pattern, so that only aircraft located at the intersection of the suppression nulls will be able to reply to a subsequently transmitted narrow-beam interrogation signal. In this way, the directivity of the interrogation processes is said to be artificially sharpened without the need for physically large antennas.
In the abstract, the reply suppression technique proposed by O'Grady et al. possesses a number of distinct advantages over the discrete addressing systems described earlier. Most importantly, reply signal overlap is reduced or eliminated without the additional airborne hardware requirement entailed in selectively responding to specially coded interrogation signals. The manner in which this technique is implemented by O'Grady et al., however, would be unmanageable in a large-scale satellite-based air traffic control system. For example, while it may be possible to produce an interfering suppression signal radiation pattern with the required precision from a number of antennas spaced around the periphery of a small area such as an airport, as envisioned by O'Grady et al., it would be difficult or impossible to do so for a large area of the earth's surface from satellites in high orbits. Even assuming that this could be done, it would still be necessary to steer the beam pattern over the area covered on a periodic basis, which would tend to render the cycle time of the system intolerably long. This derives from the fact that the selectivity of the system for individual aircraft is spatial in nature, so that it becomes necessary to repeatedly scan through a sequence of discrete spatial segments in order to obtain complete coverage (in the discrete addressing systems, by analogy, selectivity for individual aircraft is defined in terms of aircraft identity, so that it is necessary to repeatedly scan through a sequence of discrete identity codes to obtain complete coverage). Implementation of the O'Grady et al. system would require, moreover, that two different types of signals be produced, one for interrogation and one for reply suppression, thereby introducing an additional and undesirable level of complexity into the system as well as a potential source of unreliability.